Overview: Aging profoundly disturbs clearance (reabsorptive) transport across the blood-brain barrier (BBB) and blood-CSF interfaces. Consequently, neuronal health and function are impaired by catabolite toxicity, e.g., beta amyloid (Ap), in the interstitial fluid (ISF) and CSF. Detrimental ISF-CSF interactions may exacerbate aging as well as normal pressure hydrocephalus (NPH) and Alzheimer's disease (AD). Working hypothesis: Decreased CSF turnover in senescence, in the setting of altered BBB transport, leads to Ap retention in the CNS. Attenuated expression of the Ap-clearance transporter, the low density lipoprotein receptor-related protein-1 (LRP-1), in the BBB in early aging triggers a pathophysiological cascade promoting retention of toxic Ap40 and Ap42 in the brain and choroido-meningeal tissues. Thus the clearance abilities of the choroid plexus (CP)-CSF-arachnoid nexus are reduced, leading to a progressive buildup of Ap fragments at perivascular 'barrier'interfaces and in the brain interstices. Experimental paradigm: We will explore age-related functional relationships among CP and BBB Ap transporters, CSF dynamics and brain Ap burden. Brown-Norway/Fischer rats at 3,12, 20, 30 and 36 mo will be used to: i) measure total Ap in brain as a function of age and describe its spatial distribution, ii) quantify the kinetics of clearance of tagged Ap from the CP-CSF in aging and after flow inhibition induced by acetazolamide or FGF2, and Hi) characterize the age-modified expression of the Ap transporters, LRP-1 and the receptor for advanced glycation end products (RAGE), in CP and BBB, as the CSF system continually deteriorates. CSF formation rate/volume will be determined by dilution of blue dextran in ventriculo-cisternal perfusion. Age-related changes in CP transport of tagged Ap and CSF turnover rate will be analyzed by ANCOVA and multiple range and tested for associations with alterations (brain and meninges) in Ap fragments, LRP-1 and RAGE. Peptides/proteins will be assessed by IHC, ELISA, Western blots and real time PCR. Spatial, temporal and kinetic (transport) data will elucidate pathological sequences leading to deficient peptide clearance from the brain via the senescent CP-CSF-BBB system. Translation: Ap is an expedient marker to analyze aging effects on ISF-CSF relationships, and the compromised solute clearance by malfunctioning barrier systems. This sheds light on neurodegeneration.